This project will investigate the mechanisms of translational control of mRNA selection on E. coli ribosomes. The project has two parts; one is aimed at understanding the nature of the polynucleotide binding properties of ribosomal protein S1, and the other is directed at determining the role of protein S1 in the physiologic regulation of translation in bacteriophage T4-infected cells. Earlier experiments showed that ribosomes from T4-infected cells have reduced activity for the translation of f2 RNA as compared with T4 mRNAs. Treatment of control ribosomes with N-ethylmaleimide (NEM) produces the same effect, which results from the formation of NEM-S1. NEM-S1, unlike S1, binds polynucleotides very weakly. The polynucleotide binding specificity of S1 and NEM-S1 will be studied. In particular, the specificity for fMet-tRNA and poly C will be analyzed to determine the function of the two polynucleotide binding sites of S1. Direct evidence for the binding of fMet-tRNA to both S1 and ribsome-associated S1 will be sought using photochemical crosslinking methods. The mechanism for the interference activity of NEM-S1, possibly the consequence of protein-protein interactions, will be tested by trying to crosslink NEM-S1 to other ribosomal proteins or inititation factors. An in vivo function of S1 in translational control may result from the movement of S1 from ribosomes or membranes to the cytosol. The distribution of S1 in several subcellular fractions in control and T4-infected cells will be measured using a quantitative radioimmune assay. The relationship of S1 to translational control induced by reg B T4 mutants will be examined in vitro by following the synthesis of dihydrofolate reductase, which is overproduced as a result of this mutation. The RNA-dependent synthesis of beta galactosidase, or beta-galactosidase alpha-peptide, will be used to assess the function of S1 or other T4-induced components in restricting the translation of lac mRNA.